There may be some files or folders in your trash that you don't own and so can't empty from the trash. Open a terminal and run the following command, to change ownership of everything in the trash to yourself. You should then be able to empty the trash. You can copy text to/from the terminal, see its Edit menu.

Edit: you may also try overthetop's command. When I tried that just now it did delete files in my trash, but when I then opened my trash in the file browser the files were still there. Hence my more cautious approach. overthetop's command may work fine, but you may have to log out and log in again to clear any confusion your file browser may have over the state of the trash.

Thanks for the quick responses.Vincet, your command errored out while overthetop's command didn't do anything. After looking at your explanation again it seems I'll have to log out/in for the changes to take effect but I don't want to do that yet

If you have deleted the files in your trash folder, but they still appear in your file manager--you probably at least have to logout/login to get a new session for the file manager with the updated status of your trash folder..

You could also crash/restart the Xserver (ctrl alt bksp key combination); it should require a re-login of the session..